Find graceful description when missing disk.

Certain volumes (like internal storage) don't have a corresponding
DiskInfo object, so we need to fall back to using the VolumeInfo
description instead.

Bug: 77991425
Test: atest com.android.settings.ui.StorageWizardTest
Change-Id: I92d377035b6028dd31527100da54bfb1d1828ae9